This is a proposal to investigate the sequence of electron flow associated with cyanide-resistant respiration in plant mitochondria. Cyanide-resistant respiration occurs in a wide range of plants and, to a lesser extent, in many animals, but the exact path of electron flow is not fully known. There is much conflicting information in the literature concerning the role of ubiquinone and a specific iron-sulfur protein, center S-3 of succinate dehydrogenase, in the cyanide-resistant pathway. The use of the two site-specific inhibitors, dibromothymoquinone and carboxin, coupled with detailed kinetic analyses offers an opportunity for studying the role of ubiquinone and center S-3 in cyanide-resistant electron transfer. This proposal describes a series of kinetic experiments using steady state, stopped flow, and rapid freeze EPR techniques designed to elucidate the exact nature and site(s) of action of ubiquinone and center S-3 in the plant mitochondrial electron transfer chain.